Battery-powered devices (e.g., consumer electronic devices, electric and hybrid automobiles, etc.) are charged from a power source (e.g., AC power outlet) through a charging device. The charging device couples the battery to the power source through an adaptor. The cord extending between the power source and the battery-powered device can take up space. In situations where multiple devices require charging, each with their own charger and cord, the charging area can become cramped and inconvenient.
Approaches are being developed that use over-the-air or wireless power transmission between a transmitter and a receiver coupled to the electronic device. Wireless power transmission using inductive coils is one method considered as an un-tethered method for transferring power wirelessly through a coupled electromagnetic field. In wireless power transmission, power is transferred by transmitting an electromagnetic field through a transmit coil. On the receiver side, a receive coil may couple with the transmit coil through the electromagnetic field, thus, receiving the transmitted power wirelessly. The distance between the transmit and receive coils, at which efficient power transfer can take place, is a function of the transmitted energy and the required efficiency. The coupling coefficient (k) is a function of the distance between the coils, the coil sizes, and materials. The power conversion efficiency (e.g., coupling factor, coupling quality) may be significantly improved if the coils are sized and operated at such a frequency that they are physically within the so-called “near-field zone” of each other.
Wireless power systems are generally intended to operate in a frequency range substantially near (e.g., exactly at) the peak resonance of the resonant tanks of the wireless power devices. The operating frequency of a wireless power transmitter may be determined by the switching frequency of the gate drives of the bridge inverter used to convert a DC signal to the AC signal used to generate the wireless power signal. The faster the gates of the switches are switched, the faster the rate of change (dv/dt) of the switching voltage exists on the switching nodes between the switches. An electrical node that has a relatively fast rate of change (dv/dt) may capacitively couple easily to surrounding circuitry. Through such parasitic capacitive coupling, the faster the rate of change (dv/dt), the more current will flow in undesirable places (e.g., the leads of the wireless power transmitter, other components of the system, etc.). As a result, electromagnetic interference (EMI) may be introduced into the system. Conventional wireless power transmitters may introduce filters between the switch controller and the gates of the switches to slow down the gate drives to the switches of the bridge inverter. Slowing down the rate of change (dv/dt) for the gate drives of the switches may come at the expense of power loss in the switches.
Thus, in a wireless power transfer power stage there are oftentimes conflicting requirements for low noise emissions and high operational efficiency. Low noise emissions may require the switches of a conventional power transfer stage to switch relatively slowly such that the rate of change (dv/dt) of the switching voltages are not very high, which may come at the expense of power loss in the switches—both when being enabled and disabled. On the other hand, high efficiency of a conventional power transfer stage may require that the switches are switched relatively fast, but which may result in fast rate of change of the switching voltages and increased noise.